It is a documented fact that the performance of conventional two-electrode precipitators can be improved by pulse energization where high voltage pulses of suitable duration and repetition rate are superimposed on an operating DC-voltage.
The improvements obtained by pulse energization as compared with conventional DC energization are caused by the combined effect of the following advantages:
Higher peak voltage without excessive sparking, and therefore improved particle charging.
More effective extinguishing of sparks and better suppression of incipient back corona.
The corona discharge current can be controlled by pulse repetition frequency and pulse amplitude. This allows the precipitator current to be reduced below the back corona onset level in case of high resistivity dust without reducing precipitator voltage.
For short duration pulses, the corona discharge takes place well above the corona onset level for constant DC voltage and is suppressed during the remaining part of the pulse by space charges. This results in a more uniformly distributed corona discharge along the discharge electrode.
Furthermore, corona discharges from short duration pulses are less influenced by variations in gas and dust conditions. This improves the internal current distribution of a separately energized field.
Stable corona discharge is obtainable from surfaces with larger diameter curvatures. This permits the use of large diameter discharge wires or rigid type discharge electrodes with comparatively short and blunt tips, reducing the risk of discharge electrode failures.
The improvements found in precipitator performance, resulting in increased particle migration velocity, particularly for high resistivity dusts, permit reduction of the collection area for new installations or improvement of the efficiency of existing installations without increase of collection area.
For practical application, automatic control of any precipitator energization system is of major importance in order to secure optimum performance under changeable operating conditions and to eliminate the need for supervision of the setting of the electrical parameters.
With conventional DC energization, commonly used control systems regulate precipitator voltage and current, and in general terms, the strategy is aimed at giving maximum voltage and current within the limits set by spark-over or back corona conditions. The possibilities of different strategies are extremely limited, since the precipitator voltage is the only parameter which can be regulated independently.
In contradistinction, pulse energization allows independent control of the following parameters:
1. DC Voltage level PA1 2. Pulse voltage level PA1 3. Pulse repetition frequency PA1 4. Pulse width
The possibility of combining the setting of several parameters enables development of highly efficient control strategies, if the phenomena taking place in the precipitator are measured and interpreted correctly.
As it is important for the efficiency of a precipitator that the DC-voltage is maintained as high as possible, a primary objective is to control this voltage to its highest permissible level, which level is determined by the permissible corona discharge current at the DC-level between pulses.
The need for a control is due to the fact that the corona discharge current is not only a function of the DC-voltage, but is also influenced by the actual application and variations in the conditions of the gas and of the dust to be precipitated.
I have invented a method of controlling these parameters to obtain an optimum functioning of a pulse energized precipitator. It will be apparent, however, that the method might also be used for conventional DC energized precipitators, only omitting the steps in the procedure related to application of pulse voltages.